1. Field of the Invention
The invention relates to a method for cleaning a blowing head for plastic films which is kept at operating temperature by a heater during the production of films. The invention also relates to a device for carrying out the method.
2. Description of the Related Art
Blowing heads are used to feed an annular die from a round melt strand as it emerges from an extruder. The blowing heads are used in designs as mono-blowing heads for one layer of melt and as coextrusion blowing heads for a number of layers. The blowing heads must be cleaned after specific production intervals. During the cleaning, the plastic must be removed completely from the blowing head. Methods for cleaning parts soiled with polymer are already known. One of the known methods is based on fluidized bed cleaning. The component to be cleaned is suspended in a bed of quartz sand. The quartz sand lies on a tuyere bottom, through which a mixture of combustible gas and air is blown. The burning gas flowing through causes the sand to undergo turbulent movements. The cleaning effect is obtained as a result of the high temperature of the sand and the abrasive action of the flow of the sand.
In the case of a further known method, the cleaning is carried out in a vacuum chamber with the exclusion of oxygen. The blowing head is heated by means of radiant heaters. Some of the polymer melt flows out of the blowing head. Subsequently, the temperature is further increased. The remaining melt decomposes as a result of cracking of the macromolecules into the monomers such as CH4, C2H6, CO, CO4, H2. The gases thereby produced are discharged via a catalyst and converted into CO2 and H2O. By contrast with the fluidized bed method, cleaning of the inner regions of the blowing head is also possible.
A disadvantage of both methods is the fact that the blowing head has to be completely disassembled before cleaning. In the case of the fluidized bed method, only the regions of the component that can be reached by the cleaning sand can be cleaned.
In the case of the second known method, there is the disadvantage that the blowing head is only heated by means of radiant heaters. The great volume in the cleaning oven in comparison with the free volume in the blowing head makes it much more difficult to reduce the amount of oxygen in the surrounding air. When disassembling the blowing head from the extrusion installation, all the additional components such as temperature sensors and band heaters must also be removed. The investment expenditure for such vacuum cleaning installations is very high.